This invention relates to a method and apparatus for measuring vibration in a drillstring. More particularly, this invention relates to a measurement-while-drilling (MWD) tool which detects and measures longitudinal, torsional and lateral vibration downhole during the drilling of a borehole.
Vibrations within a drillstring are associated with a variety of drilling problems. For example, if the amplitude of vibration is large, drilling performance is decreased and the drillpipe, drillcollar, casing and bit are prematurely worn. In addition, drillstring failures may result from increased bending induced by the vibration. The drillstring can exhibit three types of vibrational motion, longitudinal (or axial), transverse (or lateral) and torsional. Longitudinal or axial vibrations are associated with bit and kelly bounce. Transverse vibrations are associated with bending of the drillpipe. Torsional vibrations can lead to stick slip drilling problems as well. When vibrational amplitudes are high, the drillpipe is continually loaded and unloaded, leading to a deterioration of drillpipe strength. Torsional oscillations at the bit also have a large negative effect on drilling performance.
As is clear from the foregoing, it is extremely important to detect and measure the vibrational motion of a drillstring during the drilling operation. Pertinent data, such as critical speeds and high vibration levels can be transmitted to the surface. This information can then be used to determine rotary speeds and to warn of high vibration. Avoiding critical speeds increases the life of the drillstring and results in drilling efficiency. Normally, most of the drilling energy is applied to formation of the borehole. However, if operating at an undesired resonance, most of the drilling energy is absorbed by resonance and very little goes into borehole formation.
Several methods are known for detecting such drillstring vibration. For example, an article entitled "Measurement of BHA Vibration Using MWD", D. A. Close et al, (IADC/SPE) 17273 describes a three axis vibration or resonance detector system which utilizes an array of strain gages. However, three axis resonance detectors of this type suffer from certain problems and deficiencies including the inability to differentiate between lateral and torsional vibrations.
U.S. Pat. No. 4,958,125 to Jardine et al describes a vibration sensing tool which includes four accelerometers disposed 90 degrees from each other; and which measure vibrational forces radially with respect to the outer diameter of the drillstring.